Question

Four nutritional mutants in Neurospora will grow if arginine is added to minimal medium. Some of these mutants also will grow if other substrates chemically related to arginine are added to minimal medium. The growth patterns are show in the table (+ = growth and 0 = no growth). A. What is the sequence in the metabolic pathway of arginine (order of metabolites)? B. Indicate where each mutant is blocked in the pathway?

                                                                Supplements (metabolites) added to minimal medium

Mutants                 Precursor               Glutamic Semialdehyde                     Arginine                 Citrulline                Ornithine

1                                  0                                         0                                                  +                              0                             0

2                                  0                                         0                                                  +                              +                             0

3                                  0                                         +                                                  +                              +                             +

4                                  0                                         0                                                  +                              +                             +

Answer 1

Supplementation of the medium with any amino acid downstream from an enzymatic block permits growth since the pathway is intact from that point on.

The order of metabolic pathway of arginine is

Precursor ------> Glutamic semialdehyde ------> Ornithine ------> Citrulline --------> Arginine

Mutant 1: The block is at the last step

Mutant 2: The block is at second last step

Mutant 3: The block is at first step

Mutant 4: The block at second step